Hooking the two water heaters together is no problem. The problems that you might have are finding a large enough flue to handle the two vents and getting hot water to the system now handled by the upstairs water heater.

: I am remodeling and need to relocate the hot water heater that serves the upstairs bathrooms. There is a second hot water heater in the basement and there is plenty of room there for the other one. Since we are installing a large whirlpool tub I would like to have the capacity of both heaters on on one line. Is it possible to hook two heaters to the same line?